This invention relates generally to plasma jet cutting equipment, and more specifically to a novel and improved nozzle assembly suitable for use in plasma arc cutting torches.
In transferred arc plasma jet cutting equipment, a device, commonly referred to as a "torch", uses gas flow and heat generated by an electric arc to "cut" through a metallic workpiece. A direct current electrical arc and ionized gas, between an electrode (the cathode) located in the center of the torch and the workpiece (the anode), create a jet of hot plasma through a constricting nozzle located between the electrode and the workpiece. The jet has sufficient heat and force to slice through the struck portion of the workpiece.
Current state-of-the-art nozzles are constructed from an electrically conductive material, usually copper. Unfortunately, when significant electrical power is applied to the cutting operation, there can occur a phenomenon known as "double arcing", in which the plasma arc does not pass directly through the center of the nozzle orifice, but instead deflects to the nozzle wall before reaching the workpiece.
The most common technique for combating double arcing is to add a ceramic electrical insulator with an orifice between the nozzle and the workpiece. Present designs position the insulator slightly away from the nozzle to form a gap between the two components around the orifice. This provides a conduit for cutting shield gases and cooling gases or water to be introduced for such purposes as improving the quality of the plasma arc cut, cooling the nozzle to extend its life, and helping constrict the size of the cutting arc for deeper or better cuts. The size of the gap between the nozzle and insulator is a very important determinant to the quality of cut and useful lives of the nozzle and insulator. Popular designs in plasma arc torches therefore utilize a nozzle assembly of two or more components, including a copper nozzle base and a ceramic insulator. The gap between these components is carefully controlled. These designs also provide a flow path for injecting coolant water into the plasma orifice area.
To assure a good quality of cut, and a long life for the components, the orifices of the insulator and the nozzle base must remain concentric with each other at all times, and the thickness of the coolant water flow path, as determined by the gap between the nozzle base and insulator, must be maintained within very close tolerances. Heretofore, these requirements have been achieved either by permanently bonding the insulator to the nozzle base, with glue for instance, or by assembling the insulator to the nozzle base with additional components. Typically, these include a centering sleeve fitted around the outside of the insulator and nozzle base to assure concentricity, and a spacer fitted between the nozzle base and insulator to assure a proper gap for coolant water flow.
There are several significant disadvantages to the above described plasma torch nozzle assemblies. Where the nozzle assembly, the nozzle base and the insulator are permanently attached to one another, the nozzle base frequently wears out long before the insulator under normal cutting operations. On the other hand, material irregularities in the workpiece may cause the insulator to contact the workpiece accidentally and produce irreparable damage to the insulator without harming the nozzle base. In either case the torch operator must discard and replace the entire nozzle assembly. Consequently, more money is spent for replacements than is truly necessary.
In nozzle assemblies having additional detachable components, a significant disadvantage is the overall cost of producing and assembling the additional components. Also, where a centering sleeve is fitted around the outside of the insulator with an inwardly directed gripping force, it is directly in the flow path of the cooling water and therefore interferes with flow. The centering sleeve must therefore include water passage holes, gaps, notches or spaces, all of which add significantly to manufacturing costs. Another disadvantage in using additional components is the difficulty of reassembling them with the nozzle and insulator after the torch operator has replaced the worn or broken component. Replacing only one component of the assembly requires painstaking re-balancing of the various components upon each other in order to complete reassembly successfully. Consequently, more is expended at the outset to maintain a complete inventory of nozzle assemblies.